This invention relates to an on-vehicle distance measuring device for determining whether an object in front is a vehicle or not and measuring the distance to such an object by carrying out a two-dimensional scan with electromagnetic waves such as laser light. In particular, this invention relates to such a device capable of preventing erroneously identifying a vehicle in front as a road surface while recognizing it and thereby losing sight of it.
Devices of this type for determining whether an object in front is a vehicle or not and measuring the distance to such an object by carrying out a two-dimensional scan with electromagnetic waves such as laser light have the possibility of erroneously identifying reflected waves from a road surface in front as those coming from a vehicle. In view of this possibility, Japanese Patent Publication Tokkai 2003-42757 (Ref. 1) described a method of preventing such a misjudgment by recognizing the difference between distance-direction patterns of reflected waves from a road surface and those from a vehicle. For broadly preventing a misjudgment, Japanese Patent Publication Tokkai 2004-125739 (Ref. 2) described a method of shifting an electromagnetic beam to a position where the reception intensity is higher while carrying out an offset adjustment in the vertical position. Japanese Patent Publication Tokkai 11-14746 (Ref. 3) described a device adapted to adjust the beam angle of electromagnetic waves such that the beam would become parallel to the road surface being traveled upon. Japanese Patent Publication Tokkai 8-82679 (Ref. 4) described a device provided with pulse transmitting means both for a long-distance range and a short-distance range and adapted to prevent receiving waves reflected by the road surface by offsetting somewhat upward the direction of transmission for the signal transmitting means for the shorter-distance range.
Conventional devices according to Ref. 1 are capable of distinguishing a front going vehicle from a road surface if reflected waves from a reflector of the body of the front going vehicle are being detected because distance-direction patterns of reflected waves from a road surface are significantly different from those from a vehicle. When the vehicle on which the device is mounted (hereinafter referred to as the own vehicle) is accelerating or if the road condition is changing such that the orientation of the own vehicle changes, this affects the direction of transmission of the laser light. If the front going vehicle has a body type that does nor reflect light well or if it is simply dirty, the waveform pattern reflected from such a vehicle may resemble that from a road surface. In such a situation, the distinction becomes difficult to make and it becomes easier to lose sight of a front going vehicle. This situation will be explained next with reference to FIGS. 1-3.
FIG. 1 shows a situation where there is no vehicle in front and laser light 3 transmitted from a laser radar 2 mounted to a front part of the own vehicle 1 is reflected by a road surface 4 and being detected. FIG. 2 shows another situation where the laser light 3 is not reflected by the reflector 6 of a front going vehicle 5 (being a truck in this example) but is undergoing complicated reflections by its lower back portions to be detected. FIG. 3 shows still another situation where the laser light 3 is not reflected by the reflector 6 of the front going truck 5 but is being reflected by upper parts of its back portion and the back part of the driver's cab. In the situations shown by FIGS. 2 and 3, the quantity of reflected light is small because the laser light 3 is not reflected by the reflector 6 and the waveform pattern of the reflected waves expands in the direction of the distance, coming to resemble that of reflected waves from a road surface.
The problem as described above is particularly serious in the case of so-called low speed following (LSF) mode at the time of traffic congestion. (See, for example, http/www. hido.or.jp/ITS/TS/TSF/4¥_glossary.html) If the front going vehicle becomes lost in the LSF mode, a “mode cancel” condition is started and there is no more following continued. In summary, once the waveform pattern of the reflected waves comes to resemble that from a road surface and the front going vehicle is lost, it becomes impossible to follow it and hence to further continue measuring the distance to it.
Even with devices according to Refs. 2-4, it is difficult to correctly distinguish between waveform patters of reflected waves from a vehicle and from a road surface under a special situation as described above, and it was not possible to avoid losing sight of a front going vehicle.